A source of benzene and xylenes is catalytic reformate, which is prepared by contacting a mixture of petroleum naphtha and hydrogen with a strong hydrogenation/dehydrogenation catalyst, such as platinum, on a moderately acidic support, such as a halogen-treated alumina. Usually, a C6 to C8 fraction is separated from the reformate and extracted with a solvent selective for aromatics or aliphatics to produce a mixture of aromatic compounds that is relatively free of aliphatics. This mixture of aromatic compounds usually contains BTX, along with ethylbenzene.
Refineries have also focused on the production of benzene and xylenes by transalkylation of lower value C9+ aromatics with benzene or toluene to produce xylenes as increasingly important process. Chemical plants would ideally like to process as much of the heavy C9+ aromatics as possible while minimizing and potentially removing the toluene/benzene co-feed. Both transalkylation activity and dealkylation activity are important for a successful catalyst system. Transalkylation is the ability to transalkylate methyl groups to form xylenes. Dealkylation activity is the ability to dealkylate ethyl and propyl groups present on the C9+ aromatics to allow the formation of lower methyl/ring species that may transalkylate with higher methyl/ring species to form xylenes. Metal function is required to saturate olefins formed during dealkylation while maintaining the integrity of the aromatic saturations. As plants move to increased amounts of C9+ in the feed, acceptable activity and catalyst life become challenging.
It has been shown that decoupling the dealkylation activity and transalkylation activity through use of a stacked bed system improves performance dramatically. One stacked bed catalyst system is disclosed in U.S. Pat. No. 5,942,651 and involves the steps of contacting a feed comprising C9+ aromatic hydrocarbons and toluene under transalkylation reaction conditions with a first catalyst composition comprising a zeolite having a constraint index ranging from 0.5 to 3, such as ZSM-12, and a hydrogenation component. The effluent resulting from the first contacting step is then contacted with a second catalyst composition which comprises a zeolite having a constraint index ranging from 3 to 12, such as ZSM-5, and which may be in a separate bed or a separate reactor from the first catalyst composition to produce a transalkylation reaction product comprising benzene and xylene. The ethylbenzene in the feed and/or the ethylbenzene formed during transalkylation process are partially destroyed by dealkylation of ethylbenzene to benzene and ethylene.
Another stacked bed catalyst system is disclosed in U.S. Pat. No. 5,905,051 for a process for converting a hydrocarbon stream such as, for example, a C9+ aromatic compound to C6 to C8 aromatic hydrocarbons, such as xylenes, by contacting the stream with a catalyst system comprising a first catalyst composition and a second catalyst composition, wherein said catalyst compositions are present in separate stages and are not physically mixed or blended and wherein said first catalyst composition is a metal-promoted, alumina- or silica-bound zeolite beta, and said second catalyst composition is ZSM-5 having incorporated therein an activity promoter selected from the group consisting of silicon, phosphorus, sulfur, and combinations thereof. According to the '051 patent, the use of the separate catalytic stages improves the conversion of C9+ aromatic compounds and naphthalenes to xylenes and decreases the amount of undesirable ethylbenzene in the product. The ethylbenzene in the '051 product is about 3-7 wt. % of ethylbenzene based on the weight of C8 aromatics fraction of the resulting product.
U.S. Pat. Nos. 8,183,424, 8,481,443, and 9,006,125 discloses improved performance with a stacked bed system in a process for producing xylene by transalkylation of a C9+ aromatic hydrocarbon feedstock contacted with a C6 and/or C7 aromatic hydrocarbon and hydrogen with a first catalyst comprising (i) a first molecular sieve having a Constraint Index in the range of 3 to 12 and (ii) at least first and second different metals or compounds thereof of Groups 6 to 12 of the IUPAC Periodic Table of the Elements under conditions effective to dealkylate aromatic hydrocarbons and to saturate C2+ olefins formed so as to produce a first effluent. At least a portion of the first effluent is then contacted with a second catalyst comprising a second molecular sieve having a Constraint Index less than 3 under conditions effective to transalkylate C9+ aromatic hydrocarbons with said C6/C7 aromatic hydrocarbon to form a second effluent comprising xylene.
Stacked beds using at least one zeolite having MWW framework is disclosed in U.S. Pat. No. 8,163,966 in a process to produce a product containing xylenes comprising contacting a C9+ aromatic feedstock, hydrogen, and a C6-C7 aromatic feedstock with a first catalyst comprising a first molecular sieve selected from the group consisting of MCM-22 and MCM-49 and 0.01 to 5 wt. % of a first metal element of Groups 6-10 under first conditions to form a first product, then contacting at least a portion of said first product with a second catalyst comprising a second molecular sieve selected from the group consisting of ZSM-12 and mordenite and 0 to 5 wt. % of a second metal element of Groups 6-10 and under second conditions to form a second product comprising xylenes.
Others have disclosed catalysts and processes for single bed systems. U.S. Pat. No. 6,867,340 discloses a catalyst for the disproportionation/transalkylation of various hydrocarbons that consists of a carrier and a metal component supported on the carrier. The carrier comprises 10 to 80 wt. % of mordenite and/or beta type zeolite with a mole ratio of silica/alumina ranging from 10 to 200; 0 to 70 wt. % of ZSM-5 type zeolite with a mole ratio of silica/alumina ranging from 30 to 500; and 5 to 90 wt. % of at least one inorganic binder selected from the group consisting of gamma-alumina, silica, silica alumina, bentonite, kaolin, clinoptilolite, and montmorillonite. The metal component comprises platinum and either tin or lead. The catalyst enables mixed xylenes to be produced at remarkably high yields from benzene, toluene and C9 or higher aromatic compounds through disproportionation/transalkylation with a great reduction in aromatic loss. In addition, the catalyst can maintain its catalytic activity for a long period of time without deactivation.
U.S. Pat. No. 7,626,064 discloses a catalyst and a process for transalkylation of C7, C9, and C10 aromatics to obtain a high yield of xylenes. The catalyst comprises a novel UZM-14 catalytic material comprising globular aggregates of crystallites having a MOR framework type with a mean crystallite length parallel to the direction of the 12-ring channels of about 60 nm or less and a mesopore volume of at least about 0.10 cc/gram. The UZM-14 catalyst is particularly active and stable in a transalkylation process.
U.S. Pat. No. 7,553,791 discloses a catalyst composition, a process for producing the composition and a process for the conversion of a feedstock containing C9+ aromatic hydrocarbons to produce a resulting product containing lighter aromatic products and less than about 0.5 wt. % of ethylbenzene based on the weight of C8 aromatics fraction of the resulting product. The C9+ aromatic hydrocarbons are converted under the transalkylation reaction conditions to a reaction product containing xylene. The catalyst composition comprises (i) an acidity component having an alpha value of at least 300; and (ii) a hydrogenation component having hydrogenation activity of at least 300. The composition can be produced by incorporating at least one hydrogenation component into an acidity component having an alpha value of at least 300.
Even with these advances in transalkylation technology, there is a need for improved performance, particularly in a process for conversion of C8+ aromatic hydrocarbons to lighter aromatic products.